The present invention relates to an electric power protective relay and, more particularly, to a static protective relay suitable for monitoring power lines by carrying out various relay functions collectively.
It is the duty of the protective relay that in case of occurrence of a fault, such as a short-circuit or a ground fault somewhere in the power system, the protective relay detects the abnormal state which may have an adverse effect on the system and issues a command to disconnect the faulty portion from the system immediately. This objective may be attained by a single protective relay or by a combination of various protective relays.
Protective relays are roughly classified from the viewpoint of the operating mechanism thereof into two general types, i.e., electromagnetic and static relays, and the protective relay according to the present invention belongs to the latter type. The static relay is typically constituted by an input transforming circuit, a vector composing circuit, a rectangular-wave converting circuit, a determining circuit, and an output section. The operation of the static relay results in the generation or extinction of a control output, depending on detected conditions. For a better understanding of the invention, the constituent parts of the static relay will first be described in brief.
(a) Input transforming circuit
This circuit is typically provided with auxiliary transformers such as a voltage transformer, a current transformer or the like, and serves to transform the levels of the input A.C. voltage and current into levels falling within ranges suitable for the determining circuit, to shift the phases of the input data, to optimise the transient characteristics and frequency characteristics of the input data, and to provide isolation between the input circuit section and the determining circuit section.
(b) Vector composing circuit
Necessary vector values are derived from the input voltage and current data, in order to provide the characteristics of a range relay. The vector composing circuit is principally made up of adding and subtracting circuits for processing A.C. data (vector data).
(c) Rectangular-wave converting circuit
This circuit converts sinusoidal waveforms into rectangular ones. Most relays which operate on the basis of the principle of phase detection carry out the sinusoidal-to-rectangular conversion.
(d) Determining circuit
This circuit is made up of various basic electronic circuits, and performs various arithmetic operations and determinations for identifying system failures and abnormalities.
One example of such a static protective relay is disclosed in Japanese Patent Publication No. 13236/1962. In order to provide a relay function with desired characteristics by determining the phase relationship between two AC voltages, the protective relay as disclosed comprises a gate circuit which achieves on/off operations in accordance with the phase relationship between the two voltages, an oscillator which provides a reference frequency for the gate circuit, and a counter which counts the oscillator output as it passes through the gate circuit, whereby the relay operates when the count in the counter exceeds a predetermined value.
Accordingly, a static relay is generally divided into an AC circuit section which handles AC waveforms (vector data) and a DC operating section which handles DC waveforms. In the case where various relays are combined to form a multi-function relay, there can be considered two system architectures, one being a so-called centralized system which has a common AC circuit section for the respective component relays and individual DC circuits provided separately for the respective component relays, and the other being a so-called unit system in which an AC circuit section and a DC circuit are provided for each relay. Both of these systems disadvantageously need many kinds of lead wires and a large space when building up a protective relay installation.
In order to solve such problems, there has been developed digital protective relays which utilize microcomputers. In such a system, the AC circuit section is unified and the sinusoidal waveform is converted into digital data cyclically at a high speed so that a single microcomputer processes the digital data to carry out a number of protective relay functions. The principle of this type of system is that the AC input is digitalized and the data is processed for determining the operation of each relay in a high-speed time sharing fashion, and the operation is repeated at high speed in order to carry out the determination of the operation for numbers of protective relay functions. However, this system involves complicated and time-consuming processes due to the A/D conversion and also requires a high manufacturing cost.